One of the aims of chassis alignment in vehicles with two steerable axles (A1, A2) is to ensure that the directions of travel of the steerable axles is consistent with the vehicle's axis of travel when the steering-wheel is in the horizontal position. Vehicles with two steerable axles include, for example, trucks used for the transport of heavyweight goods. The axis of travel is defined by the direction of travel of a rear axle. The vehicle's chassis geometry may be measured here with the device and the method disclosed in DE 10 2008 045 307 A1. By performing a chassis alignment and determining the vehicle's steering-wheel angle Ω (angular deviation of the steering wheel from the horizontal) for the two steerable axles A1 and A2, one obtains, as a function of the steering-wheel angle Ω, the four individual toe angles relative to the vehicle's axis of travel: SpA1left(Ω), SpA1right(Ω), SpA2left(Ω), SpA2right(Ω).
From the two individual toe angles for each of the two steerable axles one obtains, as a function of the steering-wheel angle, the direction of travel δA1 of the axle A1 relative to the vehicle's axis of travelδA1(Ω)=(SpA1left(Ω)−SPA1right(Ω))/2  (1a)and the direction of travel δA2 of the axle A2 relative to the vehicle's axis of travelδA2(Ω)=(SpA2left(Ω)−SpA2right(Ω))/2  (1b)as well as the total toe GSpA1 of the axle A1GSpA1=SpA1left(Ω)+SpA1right(Ω)  (2a)and the total toe GSpA2 of the axle A2GSpA2=SpA2left(Ω)+SpA2right(Ω)  (2b)Prior to alignment, the two steerable axles (A1, A2) are in the following state:δA1(Ω)=(Ω−ΔΩ)/steering-wheel gear ratio  (3)                where ΔΩ is the angular deviation of the steering-wheel position from the horizontal (steering-wheel misalignment) when the vehicle is travelling straight ahead (i.e. when δA1=0) and        
δA1≠δA2, where |δA1|, |δA1|, |δA2|<<5° applies on account of the vehicle's being driven straight ahead onto the test bay.
On condition that |δA1|, |δA2|<<5°, it follows that, for all the following calculations, the total toe angles GSpA1 and GSpA2 are not a function of the steering-wheel position Ω. A further condition is that effects of steering-wheel play are ruled out by suitable measures (see DE 10 2005 042 446 B3) and that the steering wheel is centered and remains so for all alignments.
The two steerable front axles with the steering-wheel gear ratio (L/Üb) are correctly aligned when
                Alignment 1 leads to ΔΩ=0 and thus to δA1(Ω)=/LÜb, where δA1(Ω)=0 for Ω=0, and        Alignment 2 leads to δA1(Ω)=δA2(Ω) and thus to 0=Δ=δA1(Ω)−δA2(Ω), where Δ is the difference in directions of travel of the steerable axles A1 and A2.        
Alignments 1 and 2 are mutually independent, meaning that the sequence of the alignments 1 and 2 may also be exchanged, or alignment 1 carried out without alignment 2 and vice versa.
The total toe angles of the steerable axles A1 (GSpA1) and A2 (GSpA2) are constant under the general conditions set forth above (i.e. |δA1|, |δA2|<<5°) and are in keeping with the specified values in each case. In other words, the total toe angles do not change when alignments 1 and 2 are performed.
The alignment 1 may be performed, for example with the device and the method disclosed in DE 10 2008 045 307 A1, subsequent to chassis alignment.
To this end, the measuring units are positioned opposite the wheels of the steerable axle A1 and measurement is commenced. The mechanic sets the direction of travel δA1 to zero.
Subsequently, the steering wheel, with which a steering-wheel balance is connected for measuring the steering-wheel angle Ω, is rotated against the steering column until the steering-wheel angle is also zero and the direction of travel δA1 remains at zero.
The disadvantage of the alignment 1 procedure is that during adjustment of the steering wheel by rotating it against the steering column, the mechanical coupling causes the direction of travel of the steerable axle δA1 to deviate from zero. The steerable axle's zero-degree position has to be restored and the steering wheel readjusted. This costs valuable cycle time.
On completion of alignment 1, the measuring units of the test bay disclosed in DE 10 2008 045 307 A1 may be positioned opposite the wheels of the steerable axle A2 and measurement commenced.
The mechanic zeroes the direction of travel δA2 of the second steerable axle A2 via the mechanical coupling between the two steerable axles. The direction of travel δA1 must remain zeroed while this is being done.
The disadvantage of this procedure for the alignment 2 is that the direction of travel of both steerable axles may change on account of the coupling, so that, following alignment of the second steerable axle, the direction of travel of the first steerable axle has to be checked once again and, if necessary, readjusted. This costs valuable cycle time.
The problem in the case of alignment 1 is that two magnitudes: the direction of travel δA1 of the first steerable axle and the steering-wheel angle Ω have to be zeroed separately.
The problem in the case of alignment 2 is the necessity of measuring the directions of travel of both steerable axles during the alignment 2 in order to ensure that the alignment is accurate.
Attention is drawn additionally to the following state of the art. DE 10 2006 036 671 A1 describes a method of determining a vehicle's axle geometry. A method is described in which lines of light are projected onto a wheel and analyzed to determine the wheel-plane orientation of this wheel.
U.S. Pat. No. 7,864,309 B2 describes a further embodiment of a method of this kind, in which not only separate points on the illumination lines are evaluated but in which, to start with, the course of a curve is determined, which represents the course of the individual lines analyzed. U.S. Pat. No. 5,675,515 describes a procedure in which the wheel plane is measured by mounting targets on the wheel whose wheel plane is to be determined EP 895 056 A2 describes a procedure in which, to determine a vehicle's chassis geometry parameters, a measuring system is assigned to each of the vehicle's wheels. U.S. Pat. No. 5,143,400 describes a procedure in which changes in the static or dynamic load distribution of a moving vehicle, as well as their influence on the toe angle, are compensated for. The intention here is to ensure that the chassis conforms to the Ackermann steering principle.